1. Field of the Invention
The present invention relates to a flow measuring device that includes a flow measuring element, such as a heater resistive element or a sensing resistive element, disposed on a surface of its support attached to a housing.
2. Description of Related Art
A conventional technology will be described below. Conventionally, an air flow measuring device including an air flow meter detachably attached to an attaching hole of an intake pipe is public knowledge. The intake pipe defines an intake air passage of an internal combustion (engine). The air flow meter includes, as illustrated in FIG. 10, a sensor body 101, a sensor support 102, and an air flow sensor. The sensor body 101 is attached such that the sensor body 101 projects into the intake pipe. The sensor support 102 is disposed inside the sensor body 101. The air flow sensor is mounted on a surface of the sensor support 102.
The sensor body 101 includes a first bypass passage 104 and a second bypass passage 105. The first bypass passage 104 takes in a part of intake air flowing in the intake air passage defined by the intake pipe. The second bypass passage 105 takes in a part of the intake air flowing in the first bypass passage 104. At an outlet of the first bypass passage 104, a passage narrowing part 106 is formed. The passage narrowing part 106 has a tapered shape and gradually decreases a cross-sectional area of the first bypass passage 104 in a flow direction of the intake air through the first bypass passage 104. Between an inlet of the second bypass passage 105 and an outlet of the second bypass passage 105, a sensor locating part 107, where the air flow sensor is placed, is provided.
The air flow sensor outputs an electrical signal in accordance with an air flow rate of the second bypass passage 105 and includes a sensor chip 108, a flow measurement element (sensing element) 109, and a controller 110. The sensor chip 108 includes a flat-plate silicon substrate. The flow measurement element (sensing element) 109 is composed of a thin-film resistive element on a surface of the sensor chip 108. The controller 110 processes the electrical signal outputted from the sensing element 109 and outputs an electrical signal to an electronic control unit (ECU). The sensor chip 108 is mounted on a sensor chip mounting area of the sensor support 102. The sensing element 109 is composed of the thin-film resistive element (e.g., a heat generating resistive element or an air temperature detecting resistive element) arranged in a predefined pattern on the surface of the sensor chip 108.
The controller 110 includes a flow detecting circuit which outputs an electrical signal (sensor output signal) in accordance with change of resistance of the thin-film resistive element composing the sensing element 109. The controller 110 further includes a temperature controlling circuit which controls a heating current flowing in the heat generating resistive element such that a temperature of the heat generating resistive element becomes higher by a constant temperature than a surrounding air temperature detected at the air temperature detecting resistive element. When a passage narrowing part is not formed in the sensor locating part 107 of the second bypass passage 105 (see FIG. 11A), exfoliation or turbulence of an air flow into the sensor locating part 107 occurs. In this case, the sensor output signal fluctuates and a measuring error is observed in an air flow measurement value. For this reason, a defect of reduction in air flow measurement accuracy occurs.
The air flow measuring device is public knowledge (see, e.g., Patent Document 1 (JP4140553 corresponding to US2005/0241386 A1) and Patent Document 2 (JP4026660 corresponding to US2003/0182998 A1)). This air flow measuring device improves the measurement accuracy of the air flow measuring device by forming a passage narrowing part 121 or 122 in the sensor locating part 107 of the second bypass passage 105 as shown in FIG. 11B and FIGS. 12A to 12C for the purpose of an attempt to limit the output fluctuation of the sensing element 109 due to the exfoliation or the turbulence of the air flow. The passage narrowing parts 121 and 122 gradually decrease a cross-sectional area of the second bypass passage 105 two-dimensionally or three-dimensionally in an air flow direction of the second bypass passage 105. Because the air flow measuring device includes the two-dimensionally narrowed-shaped passage narrowing part 121 and the three-dimensionally narrowed-shaped passage narrowing part 122 in the sensor locating part 107 of the second bypass passage 105, the output fluctuation due to the exfoliation or the turbulence of the air flow can be limited and the occurrence of the measuring error can be suppressed.
A defect of the conventional technology will be described below. The air flow measuring device described in Patent Documents 1 and 2 has a problem as follows. The output fluctuation of the sensing element 109 due to the exfoliation or the turbulence of the air flow can be suppressed by forming the two-dimensionally narrowed-shaped passage narrowing part 121 or the three-dimensionally narrowed-shaped passage narrowing part 122. However, because the cross-sectional area of the sensor locating part 107 of the second bypass passage 105 is remarkably small, flow resistance and pressure loss increase. Hence, an air flow rate (air flow velocity) in the sensor locating part 107 decreases when an air flow rate of the intake air passage is low, and a flow-rate measuring range narrows. Accordingly, the limitation of the output fluctuation of the sensing element 109, and expansion of the flow-rate measuring range are in a trade-off relationship.
In recent years, there is demand for making the engine idle even less than the existing technology for a purpose of high fuel efficiency. In this case, a wide flow-rate measuring range from a high flow rate to an extremely low flow rate is necessary, but in the air flow measuring device described in Patent Documents 1 and 2, the expansion of the air flow measuring range to the low air flow rate region is limited. Moreover, in an air flow measuring device according to Patent Document 3 (JP2005-128038A), along the entire passage of a bypass passage through which a part of a main air flow of an intake air passage of an engine flows, from an inlet to an outlet of the bypass passage, an outer circumferential wall surface of the bypass passage located between wall surfaces on both sides of the bypass passage in a width direction is formed into a semicircle-shaped recessed curved surface, or this outer circumferential wall surface is alternatively formed into an inclined surface on its one side, or this outer circumferential wall surface is formed into two inclined surfaces on its both sides. In the air flow measuring device disclosed in Patent Document 3 (JP2005-128038A), because the outer circumferential wall surface of the entire bypass passage has the above-described shape, limitation of output fluctuation of a sensing element is possible, but the limitation of the output fluctuation of the sensing element and expansion of a flow-rate measuring range are incompatible.